The present invention generally relates to fabrication of large composite structures and, more particularly, to automated composite lay up of large aircraft fuselage sections.
The structural performance advantages of composites, such as carbon fiber epoxy and graphite bismaleimide (BMI) materials, are widely known in the aerospace industry. Aircraft designers have been attracted to composites because of their superior stiffness, strength, and radar absorbing capabilities, for example. As more advanced materials and a wider variety of material forms have become available, aerospace usage of composites has increased. Automated tape layer technology has developed to become a widely used automated process for fabrication of large composite structures such as wing panels and empennage. Current tape layer technology has been improved to offer flexibility in process capabilities required for a wide variety of aerospace components. As aerospace industry tape laying applications achieve material lay up rates, for example, that may help control the manufacturing cost of large composite structures, new and innovative applications for tape layers may be defined, such as the automated tape lay up of large aircraft fuselage sections, for example, 15 to 20 feet in diameter.
Automated tape laying machines typically are gantry style machines that may have, for example, ten axes of movement with 5-axis movement on the gantry and 5-axis movement on the delivery head. A typical automated tape layer consists of a gantry structure (parallel rails), a cross-feed bar that moves on precision ground ways, a ram bar that raises and lowers the material delivery head, and the material delivery head which is attached to the lower end of the ram bar. Commercial tape layers are generally configured specifically for lay up of flat or mildly contoured laminate applications using either flat tape laying machines (FTLM) or contour tape laying machines (CTLM). On a gantry style tape layer, tooling (or a flat table) is commonly rolled under the gantry structure, secured to the floor, and the machine delivery head is then initialized to the lay-up surface.
FIG. 1 provides an illustration of a typical tape laying machine material delivery head 100. Delivery heads for FTLM and CTLM machines are basically the same configuration as that of delivery head 100 shown in FIG. 1. The delivery heads on commercial automated tape layers are typically configured to accept material widths of 75 mm (3″), 150 mm (6″), and 300 mm (12″). Flat tape layers typically use material in 150 mm (6″) and 300 mm (12″) widths. Contour tape layers typically use material in 75 mm (3″) and 150 mm (6″) widths. CTLM systems normally use the 3″ or 6″ wide material when laying up flat plane contour surfaces. Material 102 for tape layers generally comes in large diameter spools. The tape material 102 has a backing paper 106, which must be extracted as the prepreg (resin pre-impregnated fiber) is applied to the tool surface 108. The spool of material typically is loaded into the delivery head supply reel 104 and threaded through the upper tape guide chute and past the cutters 110. The material 102 then passes through the lower tape guides, under the segmented compaction shoe 112, and onto a backing paper take up reel 114. The backing paper is extracted and wound on a take up roller of paper take up reel 114. The delivery head 100 makes contact with the tool surface 108 and the tape material 102 is “placed” onto the tool surface 108 with compaction pressure. The tape laying machine typically lays tape on the tool surface 108 in a computer programmed path (course), cuts the material 102 at a precise location and angle, lays out tail, lifts delivery head 100 off the tool surface 108, retracts to the course start position, and begins laying the next course. The delivery head 100 may have an optical tape flaw detection system that signals the machine control to stop laying tape material 102 when a flaw has been detected. The delivery head 100 also typically has a heating system 116 that heats the prepreg materials to increase tack levels for tape-to-tape adhesion. Heated tape temperatures generally range from 80 F to 110° F.
Fiber placement is a similar process in which individual prepreg fibers, called tows, are pulled off spools and fed through a fiber delivery system into a fiber placement head, which is similar to delivery head 100 shown in FIG. 1. In the fiber placement head, tows may be collimated into a single fiber band and laminated onto a work surface, which can be mounted between a headstock and tailstock. When starting a fiber band or course, the individual tows are fed through the head and compacted onto a surface —such as surface 108. As the course is being laid down, the head 100 can cut or restart any of the individual tows. This permits the width of the fiber band to be increased or decreased in increments equal to one tow width. Adjusting the width of the fiber band eliminates excessive gaps or overlaps between adjacent courses. At the end of the course, the remaining tows may be cut to match the shape of the ply boundary. The head may then be positioned to the beginning of the next course. During the placement of a course, each tow is dispensed at its own speed, allowing each tow to independently conform to the surface 108 of the part. Because of this, the fibers are not restricted to geodesic paths. They can be steered to meet specified design goals. A rolling compaction device, combined with heat for tack enhancement, laminates the tows onto the lay-up surface 108. This action of pressing tows onto the work surface (or a previously laid ply) adheres the tows to the lay-up surface 108 and removes trapped air, minimizing the need for vacuum debulking. It also allows the fiber to be laid onto concave surfaces.
A fiber placement head, like the tape laying head, may be provided with several axes of motion, using an arm mechanism, for example, and may be computer numeric controlled. The axes of motion may be necessary to make sure the head 100 is normal to the surface 108 as the machine is laminating tows. The machine may also have a number of electronic fiber tensioners, which may be mounted, for example, in an air conditioned creel. These tensioners may provide individual tow payout and maintain a precise tension. The head 100 may precisely dispense, cut, clamp, and restart individual prepreg tows.
In the quest to automate the placement of composite materials at a high rate—to make the use of composites economical compared to conventional methods of fuselage fabrication—efforts have been focused at wrapping around a male mandrel, also referred to as “tool”. Today's composite, fiber material placement processes and equipment have used male mandrels exclusively, wrapping tape layers on the outside surface of the tool—such as tool surface 108. One problem with this approach is that controlling the outside surface—also referred to as the outside mold surface or “outside mold line” (OML)— of the part—such as a fuselage section—is not possible without transferring the part to a female tool or clam-shell type tooling.
Other problems with the approach of wrapping composite materials around a male mandrel, or tool, are related to the handling of large rotating tools. Both filament winding and fiber placement typically utilize mounting the tool on a horizontal spindle. The fact that the tool inertia is reacted though a relatively small spindle shaft imposes dynamic instability problems in the tool design. The typical horizontal layout also causes sag in large tools. The longer and larger in diameter the tool is, the more severe the dynamic stability and sag issues become.
As can be seen, there is a need for fabrication of composite parts using an automated lay up machine that allows material placement directly to an outside mold surface, from inside a tool, allowing greater control and accuracy forming the exterior surface of the part. There is also a need for fabrication of composite parts using an automated lay up machine that eliminates expansion and transfer of the part to another tool, resulting in less defects and higher surface quality for the part. Moreover, there is a need for an automated lay up machine for composite fabrication of large diameter fuselage sections that avoids the problems associated with horizontal tools.